1. Field of the Invention:
The present invention relates to endosteal fixation of a substitute ligament and, more specifically, to arthroscopic endosteal fixation of a substitute anterior cruciate ligament using a tapered bioabsorbable interference screw.
2. Description of the Related Art:
When a ligament becomes detached from a bone, surgery usually is required to reconstruct the ligament. Often, a substitute ligament or graft is secured into bone tunnels to facilitate incorporation and permanent attachment.
Various methods of graft attachment are known, including the use of interference screws to secure the graft against the walls of a tunnel drilled through the tibia and a socket formed in the femur. A strong graft attachment is obtained by using a metal interference screw to wedge a graft bone block to the wall of a graft tunnel formed through the bone, as disclosed in U.S. Pat. No. 5,211,647 to Schmieding. If a bioabsorbable interference screw is used, the graft is often wedged directly against the bone by the screw, without a bone block.
Bioabsorbable interference screws are usually sized so that they are slightly larger that the diameter of the tunnel, so that they dilate the bone tunnel upon insertion. Dilation advantageously compacts the soft cancellous bone between the ends of the tunnel, providing better fixation. Conventional straight-sided bioabsorbable interference screws have an interference fit of about 1 mm., i.e, about 1 mm. of bone is dilated as the screw is inserted into the bone tunnel. Although it would be desirable to use larger diameter screws for increased fixation strength, larger screws have larger tips and are more difficult to align and insert correctly. Accordingly, a need exists for a bioabsorbable interference screw which provides increased dilation and interference fit without increased difficulty of insertion.
The present invention overcomes the disadvantages of the prior art and achieves the foregoing objectives by providing a tapered, elongated bioabsorbable interference screw, the taper of the screw extending along substantially the entire length of the elongated threaded screw. The taper of the bioabsorbable interference screw of the present invention advantageously facilitates insertion of the tip of the screw, while providing superior fixation resulting from a progressively increasing diameter. Upon insertion, the bioabsorbable interference screw of the present invention fills all but 5-10 mm. of the length of the tunnel, thereby providing increased fixation strength while also promoting healing.
The tapered bioabsorbable interference screw of the present invention includes a head provided with a specially designed Delta drive socket for receiving a Delta drive screwdriver or a traditional hex-head screwdriver. The unique drive socket of the interference screw of the present invention optimizes the torque capacity of the screw. To maintain ,wall thickness, the drive socket is tapered in correspondence with the tapered outer profile of the device. The taper also permits easy insertion of the tip and shaft of the Delta driver or hex driver (also tapered) into the fixation screw.
The tapered bioabsorbable interference screw of the present invention is preferably threaded along substantially the entire length of the screw to maximize fixation strength within the tunnel. Preferably, the distal end of the screw, the end closest to the joint, has a smooth, rounded tip profile so as to minimize abrasion with the graft.
The interference screw of the present invention may be optionally provided with a cannulation for insertion over a guide pin. In this embodiment of the invention, a cannulated Delta drive or hex drive screwdriver is used to insert the screw into the tunnel over the guide pin
The bioabsorbable interference screw of the present invention is preferably formed of highly crystalline poly-(L-lactic acid) (PLLA) compound.
In the preferred method of ACL reconstruction of the present invention, the graft, preferably a hamstring tendon autograft or allograft, is secured, preferably by interference screw fixation, in a femoral socket formed through the tibial tunnel, as described, for example, in U.S. Pat. No. 5,320,626, the disclosure of which is incorporated herein. The hamstring graft is then drawn taut and secured in the tibial tunnel by insertion of the tapered bioabsorbable interference screw of the present invention. If the interference screw is fully cannulated, a guide pin may optionally be employed to guide the interference screw during delivery and installation.